The brainstem ascending neurotransmitter system interplay induces an intricate, orderly sequence of sleep stages. EEG analysis of sleep staging N1, N2, N3, and REM sleep is determined by various rhythms and frequency analyses. For example, the awake EEG features alpha (8–13 Hz) and beta (14–25 Hz) rhythms. During sleep, theta (5–7 Hz) and delta (0.5–4 Hz) predominate, although sigma (12–16 Hz) and gamma (30–80 Hz) may also be identified. The average night sleep of eight hours contains six cycles of
~90 minutes duration each, with the cycles usually following in an orderly sequence, but with a progressive lengthening of REM in the latter part of the sleep period. Sleep stages are primarily classified by the EEG, but also take into account the recordings of eye move- ments by electro- oculography and limb movements by electromyography. During NREM sleep, slow waves (theta and delta waves) originate in the frontal lobes and propagate posteriorly. In REM sleep, gamma waves (30–80 and 80–150 Hz) arise which are similar to the waking frequencies involved with attention and working memory processing. The gamma waves connect different brain regions and are the likely mechanism whereby link- ing to many different sensory domains during REM sleep occurs, as well as activation of the medial prefrontal regions and the amygdala. The amygdala and related fear circuitry are involved in anxious dreams [6]. During REM sleep, atonia prevents skeletal muscle activity, preventing dream enaction.
This complex sleep staging and its benefits were forged in our evolutionary past.
Although mammalian sleep has profound variations among species, there are some spectacular examples that shed light on human sleep; perhaps the most significant event in our history was the tree-to-ground transition. This occurred around the time of Homo erectus ~1.8 mya, and had decisive cognitive sequelae and is viewed as a major attainment in human evolution. This event necessarily coincided with the use of fire and tool production [7]. A most notable cognitive enhancement was that of episodic memory, which is a critical function of sleep, especially with respect to consolidation of memory with the newly formed, labile memories transferred into existing memory networks [8]. The improved sleep and accompanying stages also augmented REM sleep and dreaming, providing a neural podium that in turn fostered innovation and creativ- ity [7]. Together the sleep electrophysiological events sculpt the brain during a 24- hour cycle, with the slow NREM phases pruning new neural connections and the subsequent dreaming REM forming new connections relevant to the recent daily experiences. The pervasive slow- wave activity during NREM sleep can be viewed as communication chan- nels between distant brain regions that process new informational items for storage into long- term memory. Certain parts of the brain are up to 30 percent more active during REM sleep. These are processes during which information relating to autobiographical memories, emotions, and socially related experiences are updated rather than process- ing external information that occurs during the waking hours. The sometimes bizarre visual auditory and somatosensory aspects of dreams woven together in an intriguing metaphorical nature, specific to the person, are a glimpse into the workings of this infor- mation processing, which is geared toward integrating new concepts and making sense
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of interpersonal and social intricacies. The concept of emotional intelligence, which encompasses the regulation of our own emotions and the recognition in others and how we interact with them, is regarded as a key feature and function of REM dreaming.
Improving our ability to recognize emotions in others, their signals from certain eye movements, facial expressions, and body gestures, all combine for more rational and more appropriate decision- making, improving collaboration and fostering alliances, the essence of emotional intelligence.
REM sleep also facilitates the updating of information by integrating new experiences with the repository of the person’s long- term storage of individualized memories. This process stimulates creativity; the newly formed memories and the entire cache of one’s autobiography are amalgamated by wide- ranging neuronal ensembles that link dispar- ate brain regions. Novel solutions to obdurate problems, with sometimes revolutionary approaches and solutions, emerge after REM sleep and dreaming. Many examples in history of creative solutions after a good night’s sleep have been reported by engineers, artists, musicians, and scientists, with their creative solutions emerging after a specific dream. August Kekule’s snake, biting its tail, and his revelation of the circular chemical ring structure of the benzene molecule is a good example [9].
Both the augmentation of emotional advancement or intelligence and creativity, engen- dered by REM sleep, are regarded as major factors in the remarkable human cognitive evolution. Matthew Walker’s groundbreaking research and insights into sleep prompted him to describe dreaming as a “creative incubator in which the brain will cogitate vast swaths of acquired knowledge and extract overarching rules and commonalities – the gist. We awake with a revised Mind Wide Web that is capable of divining solutions to previously impenetrable problems. In this way REM dreaming is informational alchemy.”
He uses a very apt term, “ideasthesia,” seen as a pivotal factor in human cognitive and behavioral evolution [10].
The waking brain neurochemical and electrical processing comes at a price, with the high- level metabolism requiring disposal mechanisms, which is accomplished through the glymphatic system. This system mediates a marked increase in metabolic refuse elim- ination, up to 20- fold. This is accompanied by an approximate 60 percent dilated inter- stitial space and contraction of the neuroglial cells, aiding the exchange of cerebrospinal fluid (CSF) within the interstitial fluid perfusion for clearance of many products, notably amyloid and tau proteins.
Amyloid-β production, increased by awake- state neural activity, is flushed out during sleep [11]. The deposition of amyloid, for example, begins in the medial frontal lobes, the origin of NREM electrical waves. People with sleep- related problems are more likely to develop neurodegenerative disease such as Parkinson’s and Alzheimer’s disease within a four- year period, as reported in the SHARE trial [12]. In general, sleep disturbances are one of the earliest symptoms associated with Alzheimer’s disease. Sleep deprivation studied in mice was associated with a marked rise in amyloid- β deposition concentra- tion [13]. Sleep deprivation interferes at a molecular level, causing amyloid- β deposition.
Healthy middle- aged men with cerebrospinal fluid of amyloid- β42 measurements and polysomnographic evaluation revealed a 6 percent decrease in levels after one night of good- quality sleep, not seen in those with sleep deprivation. Notably, with the default mode network being the most active region in the awake state, it happens to be the loca- tion of maximum amyloid- β accumulation. Chronic sleep disturbance escalates brain amyloid- β42 and overall risk of Alzheimer’s disease and other dementias [14,15].
People with PTSD present with daytime flashbacks, night- time nightmares, disturbed REM sleep, and obtrusive memories, for example, and are at increased risk for demen- tia. Their experiences have left them with elevated noradrenaline levels that counter the development of normal and sustained REM sleep and dreaming. Their nightly emotional experiences and traumatic memories cannot be reconciled by dreaming on account of their disturbed neurochemical milieu. So far, the only interventions with some success include the noradrenaline blocker prazosin, and meditation.
Our current approaches, with widespread use of hypnotics, are therefore problematic in that they cause an impairment of the sleep architecture with an approximate 50 per- cent compromise in brain cell connections formed in learning processes. In addition, meta- analyses reveal an excess of mortality due to stroke, cardiac infarcts, and obesity, as well as an overall increase in mortality of up to 5.3 OR when using more than 132 pills annually [16].
The objective benefit has also been determined to be no better than placebo. In Walker’s phraseology, “Ambien laced sleep became a memory eraser rather than engraver” [10].
The Concept of Criticality and Sleep
Physics insights into the neuroscience of sleep come from the concept of criticality and sleep. Criticality relates to a physics principal often observed in the natural world, fea- turing the interaction of very large numbers of components. Some examples include water transformation from liquid to steam, grains in collapsing sand piles, and flocks of migrating birds. All of these examples have points of criticality at which an “avalanche”
of activity is suddenly precipitated. Cerebral networks are yet another example and are hypothesized to operate close to a critical point, the function being the enhancement of its information- processing ability. Such mechanisms confer flexibility to tackle unpredict- ability, often encountered in the natural environment, and so enabling these biological systems to reorient and adapt more rapidly [17]. Large, interconnected neuronal systems are presumed to operate near such a critical transition zone, known as resting- state net- works, and these complex systems operate near a critical point, finely balanced between volatility and stability [18,19]. There is also a criticality role of sleep and dreaming func- tion, whereby these serve as a protective mechanism to counter “super- critical behavior”
that would be injurious to brain function. The molecular processes involved, as already noted, are the synaptic pruning and plasticity rendered during sleep and dreaming for optimization of the brain’s future responses [20,21].
Recommendations for Improved Sleep and Sleep Hygiene
1. Respect the 24- hour solar light cycle with set, regular hours for retiring and waking.
2. Abide by your chronotype and juxtapose as far as possible with work schedules.
3. Prioritize night- time sleep hygiene with limitation of pre- bedtime electronic, bright light, and excessive noise exposure. Ensure low ambient temperature and minimize noise. Use low- intensity, background “white noise” apps if needed.
4. Avoid activities that impair sleep, such as high- level cognitive activity (endogenous).
Avoid the extension of the work schedule by correspondence such as emails (may increase cortisol release).
5. Avoid caffeine- containing stimulants such as coffee, energy drinks, soft drinks, or chocolate for several hours prior to retiring. Avoid substance abuse.